A Bayesian Examination of Time-Symmetry in the Process of Measurement

  • Abner Shimony


We investigate the thesis of Aharonov, Bergmann, and Lebowitz that time-symmetry holds in ensembles defined by both an initial and a final condition, called “pre- and postselected ensembles”. We distinguish two senses of time symmetry and show that the first one, concerning forward directed and time reversed measurements, holds if the measurement process is ideal, but fails if the measurement process is non-ideal, i.e., violates Lüders’s rule. The second kind of time symmetry, concerning the interchange of initial and final conditions, fails even in the case of ideal measurements. Bayes’s theorem is used as a primary tool for calculating the relevant probabilities. We are critical of the concept that a pair of vectors in Hubert space, characterizing the initial and final conditions, can be considered to constitute a generalized quantum state.


Hilbert Space Measurement Process Input State Ideal Measurement Dirac Bracket 
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  1. Aharonov, Y., Bergmann, P. G., and Lebowitz, J. L.: 1964, ‘Time Symmetry in the Quantum Process of Measurement’, Phys. Rev. 134B, 1410–1416. Reprinted in Quantum Theory and Measurement, J. A. Wheeler and W. Zurek, eds. (Princeton University Press, Princeton, NJ, 1983), 680–686.MathSciNetCrossRefGoogle Scholar
  2. Aharonov, Y. and Vaidman, L.: 1991, ‘Complete Description of a Quantum System at a Given Time’, J. Phys. A 24 2315, 1991.MathSciNetGoogle Scholar
  3. Lüders, G.: 1951, ‘Über die Zustandsänderung durch den Messprozess’, Annalen der Physik 8, 322–328.zbMATHGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 1997

Authors and Affiliations

  • Abner Shimony
    • 1
  1. 1.Istituto di StatisticaUniversità Degli Studi di GenovaGenovaItaly

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